1. Field of the Invention
This invention generally relates to semiconductor manufacturing equipment and, more particularly, to a system and method used for the processing of semiconductor wafers. 2. Description of the Related Art
During the processing of semiconductor devices, it is highly desirable to accurately control the thermal treatment to which the devices are exposed during processing. In some instances, batches of devices, primarily wafers, are processed in a semiconductor processing furnace, which has a carefully controlled processing environment to effect the desired processes.
Of importance to the present invention are processing furnaces, which have vertically arranged wafer arrays and processing chambers. In one example, a vertically arranged thermal processor includes a vertically adjustable furnace assembly and process tube. The process tube, constructed from a quartz bell jar, is vertically moveable in up and down directions within a supporting framework in conjunction with a likewise moveable furnace assembly. Heat is supplied to the thermal processor by controlling operation of heating elements within the furnace assembly. To cool the process tube within the thermal processor, the operation of the heating elements is regulated, such that interior heat is dissipated to the exterior of the processor by convection.
Unfortunately, it has been found that in most conventional vertically arranged furnaces, the desired temperature is obtained in the process tube by surrounding the tube with heating elements. In this arrangement radiation and convection are used to heat the process tube first, before the heat energy is transferred to the cooler wafers. Convective heating is usually not effective because the process tube is typically isolated. During low temperature wafer processing operations, this type of heating arrangement is usually very inefficient and time consuming. Further, in this type of arrangement, a separate cooling chamber or blower for fast cooling of the entire furnace, is typically required since, it is difficult and inefficient to lower the temperature of the entire furnace assembly between processing operations on different batches of wafers.
For the above reasons, what is needed is a system and method for isothermally distributing a temperature across the surface of a semiconductor device which provide an accurate dynamic control of the process temperature, without degradation in uniformity of the processed wafers.
The present invention provides a system and method for isothermally distributing a temperature across a semiconductor device during processing. The present invention provides a furnace assembly, which includes a processing chamber. The processing chamber can include a processing tube, such as a quartz bell jar or similar process tube, which defines an internal cavity configured to removably receive a wafer carrier or wafer boat having a full compliment of semiconductor wafers. The quartz tube is substantially enclosed in a thermal insulation material to reduce heat loss to the outside of the process chamber.
As described in greater detail below, the internal cavity of the process chamber is accessible through an opening at the bottom end of the process chamber. The opening allows the wafer carrier or wafer boat to place into the confines of the process chamber.
The wafer carrier can be mounted to an actuatable heating assembly. The heating assembly is capable of being raised into and lowered from the process chamber. When in a closed position the heating assembly seals the process tube to allow for the evacuation of the process tube in preparation for processing.
The heating assembly can include a resistive heating element, which is positioned to heat air or other gases allowed to enter the process chamber. The heating assembly can also include a reflector device, which can direct gas flow around the heating element and through the process tube.
In operation, the wafer carrier and heating assembly are vertically raised into a position within the process chamber using an actuation mechanism. Once the heating assembly forms a seal with the process tube, the process tube is exhausted or purged of air. Gas is allowed to flow into the process chamber through an inlet. The gas exchanges heat with the heating element. As the gas is made to circulate through the process tube. The reflector acts to cause the gas to circulate from the bottom end of the process tube back to the top end of the process tube. The uniform convective heating uniformly raises the temperature of the wafers.
After the wafers are processed, the heating element is powered down and a non-heated gas is allowed to flow through the process tube. The non-heated gas cools down the wafers before their removal. Once cooled, the actuation mechanism is used to remove the wafer carrier and heating assembly from the process chamber.
Advantageously, all heating and cooling of the wafers occurs internal to the process chamber using a convective type of heating. The heat transfer between the circulating heated gas and the wafers is more direct than using radiation or conduction heating arrangements. Because the wafers are cooled within the process chamber, a separate cooling chamber or blower is not required. Since the heating element has a small thermal mass, rapid ramp-up and ramp-down times are achievable.